1. Field of the Invention
The present invention relates to a continuous fiber nonwoven produced by heat fusion and having excellent bulkiness and high tensile strength. More concretely, it provides a continuous fiber nonwoven used for sanitary materials, engineering materials, agricultural materials, packing materials and the like.
2. Description of the Prior Art
In methods for producing nonwovens utilizing characteristics of heat fusion, there are a heat treating method of card webs comprising staple fibers and a heat treating method of continuous fiber webs. Although the latter method has an advantage that the production process is simple, the resulting nonwoven has the fault of low flexibility and low bulkiness.
Conventional continuous fiber nonwovens, which are produced by a method of heat fusion and used for sanitary materials, engineering materials and the like, are mainly made of fibers of one component, since such fibers do not develop crimps, they have low bulkiness.
As known methods for developing the steric crimps of a spiral form (abbreviated as spiral crimps, hereinafter) in the fibers of one component, there are a method for developing the spiral crimps based on the difference of heat shrinkage inside the fiber by pulling out the spun fiber while partial quench is applied to the fiber (Japanese Patent Publication No. 45-1649), and a method for developing the crimps based on the difference of degree of crystallization by blending a nucleating agent into a certain part of the fiber cross-section (Japanese Patent Application Laid-open No. 5-209354). In the former method, however, the crimps are loosened through the heat treatment process for processing the fiber into a nonwoven and the bulkiness becomes insufficient. In both methods, since the fiber is constituted from one component, a hot pressing method is only used as the heat treatment process for processing the fiber to the nonwoven, so that the spiral crimps of the fiber is pressed resulting undesirable bulkiness.
It is known that the spiral crimps are developed in the fiber by compositely spinning several thermoplastic resins into a parallel or eccentric sheath core type arrangement (Japanese Patent Application Laid-open Nos. 48-1471 and 63-282350). In the nonwovens using these composite fibers, however, although it was recognized that the bulkiness was improved, the tensile strength was the same as (or less than) that of conventional nonwovens of one component fibers, so that more improvement has been desired.
The present invention provides a continuous fiber nonwoven having excellent bulkiness and high tensile strength in view of the above conditions of the continuous fiber nonwovens produced by heat fusion methods.
The inventor of the present invention has earnestly studied to solve the above problems by aiming at the relation between the spiral crimps developed in the composite fibers and the arrangement of components on the fiber cross-section. As a result, he has had knowledge that these aims are attained by using composite fibers comprising several thermoplastic resins arranged in a parallel or eccentric sheath core type, in which the thermoplastic resins having a low melting point is located on the outside of the spiral crimps developed by stretching the fibers, and he has completed the present invention.
Namely, the first invention of the present application provides a continuous fiber nonwoven comprising composite continuous fibers having the spiral crimp obtained by compositely spinning two kinds of thermoplastic resins having difference in melting point of 15xc2x0 C. or more, characterized in that the contact points of the fibers are adhered one another by fusing the thermoplastic resin having a low melting point and located on the outside of the spiral crimps.
The second invention of the present application provides a method for producing a continuous fiber nonwoven comprising: preparing the first thermoplastic resin and the second thermoplastic resin having a melting point 15xc2x0 C. less than that of the first thermoplastic resin and an elastic shrinkage 1% less than that of the first thermoplastic resin; compositely spinning these resins in a composite ratio of 60/40-40/60 into a parallel type or an eccentric sheath core type, in which the second thermoplastic resin is a sheath and the first thermoplastic resin is a core eccentric to the sheath; stretching the resulting yarn over 1.2 times as long as the unstretched yarn at a temperature lower than the melting point of the second thermoplastic resin; and heat treating the yarn at a temperature higher than the melting point of the second thermoplastic resin and lower than the softening point of the first thermoplastic resin to adhere the contact points of the fibers.